Aircraft engine fuel pumps have, in recent years, been required to provide higher pressures, operate at higher speeds and furnish increased performance characteristics with regard to such factors as fuel heating. Various pump stages, in turn, require higher charging pressures due to higher speeds. Hence, it is no longer possible to subject shaft seals to low fuel pressures because such low fuel pressures beget unacceptable leakage of fuel from the pump bearings to the seal cavity which occasions an excessive temperature rise in the inlet fuel. Instead, shaft seals in state-of-the-art aircraft engine fuel pumps must be referenced to the inlet pressure of a high pressure stage of the pump.
Conventional shaft seal assemblies, such as illustrated in U.S. Pat. No. 4,417,734, exhibit eminently satisfactory performance when the seal cavities are referenced to low pressures. However, when the seal cavities in conventional shaft seal assemblies are exposed to higher pressures, the annular seal face portion of the carbon face seal experiences significant wear which eventually causes leakage. The reason for such wear of the annular seal face portion is that the contigerous annular thrust face portion of the carbon face seal (which has grooves or slots therein) is well-supplied with lubricating fuel while the annular seal face portion which is exposed to same sealing pressure of the sealing face of the runner (or shaft flange) runs at a higher temperature with far less lubrication.